Latch locks of the type herein discussed are usually operated by rotation of a spindle extending from the lock case, and in the case of a mortise lock, extending through at least one side of the door. A lever, knob, handle or like door furniture is mounted on the spindle to facilitate operation of the latch. Rotation of the spindle operates a system of cams or levers which withdraws a latch bolt from latching engagement with a latch keeper or slot in the door jamb and permits the door to open. The mechanism is provided with a lock operable for example by rotation of a separate cylinder, which drives a wedge or block into a position which prevents rotation of the spindle and thus prevents retraction of the bolt until the door is unlocked thus removing the wedge or block.
A disadvantage of such mechanisms is that if excessive torque or entry effort is applied to the handle or knob while the mechanism is locked then damage to the mechanism results. To avoid such damage it is commonly practiced to groove the spindle in such a way as to ensure that the spindle shears prior to other parts of the mechanism becoming damaged. While that avoids the need to replace the whole lock it remains necessary for the spindle to be replaced before the mechanism can be operated again. Often the services of a tradesman are required to replace the spindle before access can be given through the doorway.